Human sleep is generally described as a succession of five recurring stages (plus waking, which is sometimes classified as a sixth stage). Sleep stages are typically monitored using a polysomnograph to collect physiological signals from the sleeping subject, including brain waves (EEG), eye movements (EOG), muscle activity (EMG), heartbeat (ECG), blood oxygen levels (SpO2) and respiration. The commonly-recognized stages include:                Stage 1 sleep, or drowsiness. The eyes are closed during Stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept.        Stage 2 is a period of light sleep, during which the body prepares to enter deep sleep.        Stages 3 and 4 are deep sleep stages, with Stage 4 being more intense than Stage 3.        Stage 5, REM (rapid eye movement) sleep, is distinguishable from non-REM (NREM) sleep by changes in physiological states, including its characteristic rapid eye movements.Polysomnograms show brain wave patterns in REM to be similar to Stage 1 sleep. In normal sleep, heart rate and respiration speed up and become erratic, while the muscles may twitch. Intense dreaming occurs during REM sleep, but paralysis occurs simultaneously in the major voluntary muscle groups.        
Sleep apneas commonly occur in conjunction with a variety of cardiorespiratory disorders. The relationship between sleep apnea and heart failure, for example, is surveyed by Bradley et al. in two articles entitled “Sleep Apnea and Heart Failure,” including “Part I: Obstructive Sleep Apnea,” Circulation 107, pages 1671-1678 (2003), and “Part II: Central Sleep Apnea,” Circulation 107, pages 1822-1826 (2003), which are incorporated herein by reference. The authors define “apnea” as a cessation of airflow for more than 10 sec. This term is distinguished from “hypopnea,” which is a reduction in but not complete cessation of airflow to less than 50% of normal, usually in association with a reduction in oxyhemoglobin saturation (commonly referred to as “desaturation”).
Sleep apneas and hypopneas are generally believed to fall into two categories: obstructive, due to collapse of the pharynx; and central, due to withdrawal of central respiratory drive to the muscles of respiration. Central sleep apnea (CSA) is commonly associated with Cheyne-Stokes respiration, which is a form of periodic breathing in which central apneas and hypopneas alternate with periods of hyperventilation, with a waxing-waning pattern of tidal volume. CSA is believed to arise as the result of heart failure, though obstructive sleep apnea (OSA) may also occur in heart failure patients.
Both OSA and CSA increase the strain on the cardiovascular system and thus worsen the prognosis of the heart failure patient. In some cases, both types of apneas may occur in the same patient, even at the same time (superposition). Classifying respiratory events as central or obstructive is considered to be a critical point, since treatment may differ according to the type of events, as pointed out by Pepin et al. in “Cheyne-Stokes Respiration with Central Sleep Apnea in Chronic Heart Failure: Proposals for a Diagnostic and Therapeutic Strategy,” Sleep Medicine Reviews 10, pages 33-47 (2006), which is incorporated herein by reference. Both CSA and OSA can be manifested in periodic breathing patterns.
Various methods have been proposed in the patent literature for automated apnea detection and diagnosis based on patient monitoring during sleep. For example, U.S. Patent Application Publication US 2004/0230105 A1 describes a method for analyzing respiratory signals using a Fuzzy Logic Decision Algorithm (FLDA). The method may be used to associate respiratory disorders with obstructive apnea, hypopnea, central apnea, or other conditions. As another example, U.S. Patent Application Publication US 2002/0002327 A1 and U.S. Pat. No. 6,839,581 describe methods for detecting Cheyne-Stokes respiration, which may be used on patients with heart failure. The methods involve performing spectral analysis of overnight oximetry recordings, from which a classification tree is generated. Another method, based on monitoring oxygen saturation and calculating the slope of desaturation events, is described in U.S. Pat. No. 6,760,608. Yet another method for classifying sleep apneas is described in U.S. Pat. No. 6,856,829. In this case, pulse waves from the body of a patient are detected, and the envelope of the pulse waves is created by connecting every peak of the pulse waves. The normalized amplitude and period of the envelope are used in determining whether the patient has OSA, CSA, or mixed sleep apnea syndrome. The disclosures of the patents and patent applications cited above are incorporated herein by reference.
It has been suggested that sleep monitoring can be used for assessing cardiorespiratory risk. For example, U.S. Pat. No. 5,902,250, whose disclosure is incorporated herein by reference, describes a home-based, wearable, self-contained system that determines sleep-state and respiratory pattern, and assesses cardiorespiratory risk. A respiratory disorder may be diagnosed from the frequency of eyelid movements and/or from ECG signals. Cardiac disorders (such as cardiac arrhythmia or myocardial ischemia) that are known to be linked to certain respiratory disorders also may be inferred upon detection of such respiratory disorders.
Photoplethysmograph devices, known commonly as pulse oximeters, provide instantaneous in vivo measurement of arterial oxygenation by determining the color of blood between a light source and a photodetector. To determine the blood oxygen saturation, light absorption measurement is carried out at two wavelengths in the red and infrared ranges. The difference between background absorption during diastole and peak absorption during systole at both wavelengths is used to compute the blood oxygen saturation.
Photoplethysmograph signals provide information not only on blood oxygenation, but also on other physiological signs. For example, U.S. Pat. No. 5,588,425 describes the use of a pulse oximeter in validating the heart rate and/or R—R intervals of an ECG, and for discriminating between sleep and wakefulness in a monitored subject. It also describes a method for distinguishing between valid pulse waveforms in the oximeter signal. U.S. Pat. No. 7,001,337 describes a method for obtaining physiological parameter information related to respiration rate, heart rate, heart rate variability, blood volume variability and/or the autonomic nervous system using photoplethysmography. U.S. Pat. No. 7,190,261 describes an arrhythmia alarm processor, which detects short-duration, intermittent oxygen desaturations of a patient using a pulse oximeter as a sign of irregular heartbeat. An alarm is triggered when the pattern of desaturations matches a reference pattern. The disclosures of the above-mentioned patents are incorporated herein by reference.